Paper trimmers



PAPER TRIMMERS Feb. 8, 1966 3 Sheets-Sheet 1 Original Filed Jan. 15, 1959 I lllllllllllllll ll llllllllllllll IN VEN OR.

Feb. 8, 1966 suvE Ro 3,233,493

PAPER TRIMMERS Original Filed Jan. 15, 1959 3 Sheets-Sheet 2 INVENTOR.

Feb. 8, 1966 1.. SUVERKROP PAPER TRIMMERS Original Filed Jan. 15, 1959 3 Sheets-Sheet 5 mew amawop INVENTOR.

rrae Y United States Patent 3,233,493 PAPER TRIMMERS Lew Suverkrop, P.O. Box 436, Bakersfield, Calif. Original application Jan. 15, 1959, Ser. No. 786,943, now Patent No. 3,151,515, dated Oct. 6, 1964. Divided and this application Sept. 25, 1963, Ser. No. 327,856 1 Claim. (Cl. 83-694) This application is a division of Serial No. 786,943, now Patent No. 3,151,515.

This specification describes improvements applicable to paper trimmers generally, and particularly to those used for trimming photographs, blueprints, photostats, and the like. More specifically they apply to the type of print trimmer in which the work board, upon whose surface the print is held during the trimming operation, is hinged on the base of the trimmer. In this type the work board generally slopes down toward the operator as he stands before the trimmer. The hinges are at the lower, forward edge of the work board, while the opposite edge of the board is fitted with the upper of two shearing blades. This upper blade is secured at its ends to the work board with its cutting edge parallel to the higher, rearward edge of the board, and slightly over and beyond that edge. Also hinged to the base of the trimmer is the lower shearing blade, along a hinge line that is substan tially below and nearly parallel to the upper blade. The cutting edge of the lower blade engages with the cutting edge of the first blade, at one end thereof when the work board is in the up position. From the point of engagement, this cutting edge slopes downward toward its other end in such manner that shearing contact between these two blades progresses from one side to the other of the trimmer, along the blades, as the board is pressed downwardby the operator. A spring keeps the lower blade in proper contact with the upper one during the shearing process. A second spring returns the work board to its terminal up position after the operator has withdrawn this pressure from it; and a stop limits this upward motion of the board to a position where the two shear blades are apart and ready to start shearing a print placed between them. Clearly, the print may be manipulated and held throughout the trimming process since the cutting action is efiected merely by pressing downward on the work board without removing the hands from the print.

The type of print trimmer just described has been known for years. At first glance, it seems to have obvious advantage over the common, so-called photographic" type of trimmer. In this latter type the work board forms the :base of the trimmer. It is fitted with a shear blade that forms the sharp upper edge along the right side of the board as the operator views it. A second blade is pivoted at one end to the far end of the first blade. Holding the print with his left hand, he grasps the free, handled end of the second blade with his right hand, and brings it down so as to trim the print placed between the blades, cutting it along the shearing edge of the board. Over this, the advantage that immediately appears in the hinged work board type trirner, previously described, is that of speed. This is especially apparent when the upper surface of the work board is finished to have a low coeflicient of friction, thereby ofiering little resistance to movement of the print while manipulating it into position for cutting. In such cases openators quickly learn to coordinate movement-s of positioning the print and pressing the work board to effect shearing. This makes possible 2. work speed with the hinged board trimmer that is several times the maximum possible with the other type. However, hinged work board trimmers heretofore have also been characterized by offsetting disadvantages; and these have prevented this type from becoming so ice popular as it should become once the problems of these disadvantages are solved.

In addition to the objectives of the parent application, Serial No. 786,943, now Patent No. 3,151,515, it is the particular object of the features of the invention covered by this divisional application to provide an upper, moving blade construction, which is fixed to and carried by the movable work board, which will combine lightness, minimum thickness, and adequate stiffness, to reduce mater-ially the inertia of the moving work board and thereby increase trimming speed while decreasing operator fatigue.

FIG. 1 is a perspective view of such a trimmer, ready for use, taken from a point quartering on the front of same;

FIG. 2 is another perspective view of such a trimmer, ready for use, taken from a point quartering on the rear of same;

FIG. 3 is another perspective view, taken from a point quartering on the front of same, with the work board raised from the operating position so as to disclose parts concealed in FIGS. 1 and 2, and to indicate how part of the trimmer are folded for shipment or storage;

FIG. 4 is a diagram, in the form of a view from one end of the trimmer, showing the manner in which it may be folded;

FIG. 5 is a cross section along the line DD in FIG. 6;

FIG. 6 is a view in the direction of the length of the lower blade, showing construction of the device for ap-' plying spring pressure to the lower blade, and the stop for limiting upward motion of the work board;

FIG. 7 is another view of the spring pressure device shown in FIG. 6;

FIG. 8 comprises two views of the bearing in which the spring pressure device of FIG. 6 oscillates and by means of which it is secured to the base of the trimmer;

FIG. 9 shows assembly of parts by means of which adjustable spring pressure is applied to the work board to raise it after trimming;

FIG. 10 is a cross section along line C-C of FIG. 9 showing construction by means of which spring tension adjustment is made and held;

FIG. 10-A is a plan view of two of the parts shown in FIG. 10;

FIG. 11 is a plan view showing an adjustable support for the print gripper and aligning gage;

.FIG. 12 is a plan view of an auxiliary support for the print gripper;

FIG. 13 is a plan view showing the print grip actuator of FIG. 13;

FIG. 15 is a plan View from below showing the print grip actuator of FIG. 13;

FIG. 16 is a view of the support shown in FIG. 11, taken from the rear of the work board;

FIG. 17 is a view from the end of the work board showing the support seen in FIG. 11;

FIG. 18 comprises two views of the print grip actuator adjusting lever seen in FIGS. 13, 14, and 15;

FIG. 19 comprises two views of the print grip actuator adjusting lever seen in FIGS. 13, 14, and 15;

FIG. 20 is a view of parts seen in FIG. 14, taken from the rear of the work board;

FIG. 21 is a cross section taken along lines AA in FIG. 12, to show better the form of the print grip blade and the auxiliary support;

FIG. 22 is a plan view of the adjustable gage pin device;

FIG. 23 is a side elevation of the gage pin device shown in FIG. 22; and

FIG. 24 is a cross section along the lines B-B of FIG. 22.

Referring to FIGS. 1, 2, and 3, work board 1 is attached by hinges 2 on support strip 3 which is integral with base 4. Brackets 5, fixed to the rearward corners of board 1, serve for securing the upper shear blade 6 at its ends by means of screws 7. Lower shear blade 8 is attached by hinge 9 to strip 10 integral with base 4. Stop 11 is secured to blade 8 by means of screws 12 through elongated holes in stop 11 so that the upper stopp ng position of board 1 may be adjusted. Downward motion of board 1 is limited by rubber bumpers 13 on support blocks 14 integral with base 4.

To eliminate shock when board 1 is brought to rest at its upper terminal position, metallic bumper 15, detailed in FIGS. 5 and 6, is provided with rubber washer 16. Bumper 15 is free to slide in the hole provided in stop 11 where it is retained by a cotter pin or other ordinary means.

Lower blade spring device.-Heretofore in trimmers of this type, the lower blade (corresponding to numeral 3 in the accompanying drawings) was forced into required shearing contact with the upper blade 6 by means of a tension spring. One end of this spring was anchored to the base 4 of those trimmers; and the other was attached direct to the lower blade 8. So that the descending board 1 would clear it, it was necessary to attach the other end of this spring to the lower blade 8 at a point which gave only a short lever arm, measured radially from the hinge axis. Aside from that kind of spring arrangement being unsightly due to its makeshift and Rube Goldberg ap pearance, it invariably resulted in excessive flutter of the lower blade 3 every time the board 1 was depressed. This incidental flutter was not merely subjectively annoying. It is of a kind that caused undue wear in the cutting edges of both shearing blades d and 8 of those trimmers.

In the trimmer covered by this specification, that objectionable condition is corrected by a novel means of applying spring pressure to lower blade 8. The spring pressure is not direct. It is applied through a spring loaded lever 17. By this means the pressure on blade 3 is applied at a point well removed from the hinge axis and all harmful flutter is eliminated. Spring lever 17 is of round bar stock bent into the form shown in FIGS. 6 and 7. The upper end is fitted with roller 18, preferably of graphite impregnated nylon or like material. This roller is in rolling contact with the outward, rearward surface of stop'11. The lower end of lever 17 is rotatably secured to base 4 by means of bearing 19 detailed in FIG. 8. Torsion spring 20 has one end bearing against base 4 and the other end formed so that it presses against lever 17. Lever 17 is as long as possible but still of such length that roller 13 continues to bear against stop 11 when blade 8 is laid flat on base 4 as seen in FIG. 3.

Adjustable work board spring.He-retofore, in trimmers of this type, the springs employed for returning the work board to its upper terminal position varied from buggy-type springs beneath the board tolong tension springs attached to a high gallows frame above it. None of them was adjustable, apparently the expectation being that differences in operators requirements, in practical spring manufacturing tolerances, in variations in springs due to age, wear, and temperature, all were items too small to be worth considering. However, the present studies have indicated that not every one of these is of minor importance; and, while one or more of them individually may be of minor importance, in combination they very materially do affect the speed and convenience with which trimmers of this type can be operated.

As shown in FIGS. 3 and 9, work board return spring 21, one end of which reacts against pin 22 in shaft 23, is formed at its other end so as to act against the under side of lever 24 which is free to rotate on shaft 23. The free end of lever 24 has an integral pin upon which roller 24l1 is rotatably secured. Roller 24-1, through a suitable pressure plate, applies the pressure to board 1. One

end of shaft 23 is carried rotatably by support 25 secured to base 4. The other end of shaft 23, of larger diameter, is either hobbed or knurled with teeth to engage with the thread of adjusting screw 26. Suitable holes in pillow block 27 receive the adjusting screw 26 and the Worm gear formed on shaft 23, rotatably and with teeth and thread in proper mesh. In other applications where adjustment must be made while the parts are under heavy load, it may be desirable to provide a steel ball. to take the thrust of adjusting screw 26 and make it easier to turn, particularly if pillow block 27 is made of soft material such as that in many die castings. However, this is hardly necessary in the present trimmer. It has been found that when board 1 is raised to the position shown in FIG. 3 for adjusting the action of spring 2 1, that position is so far above the place where spring 21 is under load, adjustment screw as can readily be turned by hand. This can be done even when it is an ordinary headless set screw protruding but slightly above the upper surface of pillow block 27. But it has been found that a minute adjustment of screw 26 very materially affects the effi oiency of the trimmer. For this reason it is preferable that the upper end of adjust-ment screw 26 have a dishlike form with numbered graduations which can be noted against an index 28 fixed to pillow block 27 as seen in FIGS. 10 and 10A. This means is convenient to the operator and greatly speeds up the certainly of precise adjustment and readjustment of spring 21 so that it gives exactly the kind of motion in board 1, both upward and downward, which will achieve maximum speed and minimum operator fatigue. Holes in pillow block 27 provide for firmly securing it to base 4 by means of screws.

Print guide and grip.L0cated toward the operator from and parallel with blade 6 is print grip 29 shown in details of FIGS. 11, 12, 13, 16, 17, 20, and 21. This is preferably of heavy transparent plastic sheet, marked on its underside with guide lines 30 running parallel with its length. Grip 29 is attached, by means of binder head scr ws 3-1 through countersunk holes in it, to grip bar 32. Grip bar 32 is preferably of square cross section with this dimension equivalent to a common increment of distance such as inch at which increment it may be convenient to space lines 30. Moreover, it is preferable to locate the edges of grip 29 and of grip bar 32 so that each is at a convenient multiple of that increment from the cutting edge of blade 6. These features make it easy to position quickly a .print so that it will be trimmed parallel to and at a desired distance from a border or other line of the print. The edge of grip 29 nearest the operator may be turned upward as indicated in the cross section of FIG. 21 so as to funnel prints under grip 29. This construction strengthens grip 29 and makes it possible to locate grip 29 close to board 1 without in the slightest hindering the quick placement of a print for trimming. It will be noted from the cross section in FIG. 21 that the lower side of blade 6 may be relieved, giving another funnel form further to assist in the rapid placement of prints.

At its ends, grip bar 32 is round in cross section to serve as journals where it is rotatably held by grip bar supports 33 each of which is provided with a boss 34. The holes of support 33 are located at the apices of a triangle, one side of which is substantially parallel to the surface of board 1. This side extends from the pivot screw 35 to the axis of grip bar 32. A second side, prependicular to board 1, extends from pivot screw 35 to holding screw 36. The hole for pivot screw 35 provides bare clearance, while a considerably larger hole in support 33 surrounds holding screw 36. This provides for considerable angular movement of support 33 about pivot screw 35, resulting in movement of grip bar 32 up and down with respect to board 1. The adjustment is made with both screws 35 and 36 somewhat loose. Having made the adjustment, each screw 35 and 36 is tightened against its washer 37 so as to secure support 33 to its adjacent bracket 5 By action which will be explained below, immediately the operator presses board]. to shear the print, and prior to the beginning of shearing action, print grip 29 rotates about the axis of grip bar 32 so that one edge of grip 29 presses on the print and secures it to board 1. This holds the print firm against the lateral forces of shearing, while shearing proceeds. Since the print cannot move with respect to the edge of upper blade 6 during this action, the trim line will duplicate the straight form of that blade. To assist in the effectiveness with which grip 29 secures the print, it is preferable that the part of board 1 that is immediately below the gripping edge of grip 29 be surfaced with friction material 38 such as sponge rubber or abrasive sheet, as indicated in FIG. 21. It ispreferable that the remainder of the surface of board 1 be covered with material that lends itself to developing a very low coefficient of friction. Obviously, this greatly facilitates quick manipulation of the print into proper alignment. For this part 39, I have used linoleum, suitably waxed. The color of the linoleum, such as green or black, may be chosen to minimize glare.

Print grip actuat0r.The rounded section at one end of grip bar 32 is extended to carry grip actuator roller lever 40 by set screw 42. As indicated by the drawings and made clear by details of FIGS. 18 and 19, boss 43 on actuator lever 40 extends into hole 44 in adjustment lever 41. Hole 44 in lever 41 is sufiiciently larger than boss 43 to allow considerable angular movement of lever 40 with respect to lever 41 when they are mounted on their journal of grip bar 32. This provides for convenient adjustment of gripping action. To facilitate making this adjustment and to secure levers 40 and 41 to one another after the adjustment has been made, adjusting screws 45 in adjustment lever 41 have their pointed ends resting in a suitable groove 46 in boss 43.

Crank pin 47 on lever 40 carries roller 48 together with a cotter pin and washer orother means for keeping it there. A second roller 49, aligned with roller 48, is similarly located and secured on roller shaft 50 which is integral with roller shaft support 50-1. This support 50-1 is fastened to the underside of bracket 5. Crank pin 47 and roller shaft 54 are drawn toward one another by spring 51. Looped ends of spring 51 fit into suitable grooves in pin 47 and shaft 50. Preferably, rollers 43 and 49 are grooved to accommodate grip actuator cam sector 52 between them as indicated in the drawings. Sector 52 is so located on base 4 that its larger curves center on the axis of hinges 2. These facts combine to make sector 52 remain stationary when the trimmer is operated, although only pivotally held on base 4. The lower end of sector 52 is positioned and rotatably secured by shoulder screw 53 to bracket 54 which is fixed to base 4. It is preferable that ordinary means in these parts allow making some vertical adjustment of shoulder screw 53 and some lateral adjustment of bracket 54. Sector 52 is of constant thickness. But its upper part is of width which permits spring 51 to draw lever 40 into a position that makes grip 29 parallel to and clear of board 1 as indicated in FIG. 21. Below this upper part, sector 52 is wider. Its width is such that when rollers 48 and 49 span it, due to the camrning action where the narrow and wide parts of sector 52 meet, actuator lever 49 has rotated grip bar 32 enough to cause the edge of grip 29 nearest blade 6 to bear on a print beneath grip 29. As shown by the drawings, the rearward edge of sector 52 is a single, continuous curve that rests against roller 49 whose axis is stationary with respect to board 1. The opposite, forward edge of sector 52 is formed by two curves having a common center which is also the center of the curve on the rearward edge. Thus the upper part of sector 52 is of constant radial width and narrower than the lower part, also of constant width. The cam in sector 52 is formed where the narrow and the wide parts join. This is preferably in the form of a fourth radius, equal to that of the groove in roller 48 where it bears against sector 52. With this construction, movement of lever 40 and therefore grip 29 begins at the very instant that the operator starts the downward motion of board 1; and very little motion of board 1 is required for the cam of sector 52 to complete the movement of actuator lever 40 and the consequent action of grip 29 in securing the print on board 1. This makes it possible for the trimmer to secure a print throughout the shearing action, automatically; and also to leave the print entirely free for alignment manipulation before shearing begins.

Due to the slender proportions of grip bar 32, reactions from its rotation to effect gripping the print cause it to bow upward. To insure constant gripping action of grip 29 throughout its length, one or more auxiliary grip bar supports 55 may be provided. Support 55 is adjustably secured to blade 6; and its underside rests against a cylindrical surface provided on grip bar 32 as indicated in FIG. 21.

Kinetic initial shearing.-Heretofore in trimmers of this type, the shear blades 6 and 8 remained in shearing contact when the board 1 was in its terminal up position. In them, the point of shearing contact between the blades and the point in the paper where shearing would begin were coincident. Consequently, shearing began with zero velocity in the upper blade 6. Considerable static pressure frequently was required of the operator in order to start the cut although, if he could get it started, he might experience no great difficulty in continuing it. By the nature of such trimmers heretofore, in the absence of levers or other means of mechanical advantage, they were considerably limited in paper thickness they could cut, compared with the photographic type of trimmer with its lever action.

In the present trimmer, blade guide 56 (see FIGS. 2, 3, 16 and 17) makes it possible to raise blade 6 a useful distance above and out of contact with blade 8. In the absence of this guide 56, when board 1 goes to its terminal up position, blade 8 would drop against board 1 in a place below blade 6. When next board 1 was depressed, blade 8 would hang up on board 1, or blades 6 and 8 would collide. In either case, shearing could not take place. With guide 56 properly adjusted, when board 1 moves to its terminal up position, blade 8 bears against the upturned outboard face of guide 56 in such position that the next downward movement of board 1 brings blades 6 and 8 into proper shearing engagement with one another. In the interval of time between the start of the downward motion and the start of actual shearing, kinetic energy is developed. This is suflicient to permit shearing thicker paper, without additional effort on the part of the operator. This has been found to be true, even when the time interval is so small as not to be noticeable to the operator. In addition to increasing the thickness that may conveniently be cut with this trimmer, the time interval made possible by blade guide 56 further insures that the gripping action (caused by the cam on sector 52) will be completed before shearing begins.

Low inertia shear blade.0perating speed of this type of trimmer is very much affected by total weight of parts secured to board 1, and by the distance to the center of mass of this aggregate measured from the hinge axis. Even when properly counterbalanced by return spring 21, any increase in weight means an increase in inertia. This means a reduction in the speed with which it may be moved, for a given effort on the part of the operator. In this, the most important factor is upper shear blade 6. Of necessity it is located on board 1 as far as possible from the hinge axis, whereas the place where the operators hands press upon board 1 is much closer to that axis. He is at a disadvantage in causing downward movement of blade 6, and reduction in its weight is much to be desired. But a number of factors make the problem difficult. To reduce weight, the blade cannot indiscriminately be reduced in area, or made of material chosen only for low specific gravity. Blade 6.is a cutting tool and this requires characteristics of tool steel rather than those found in a light aluminum or magnesium alloy. It must be sufficiently rigid to resist lateral forces from lower blade 8, particularly those which may result in a curved trim line. Light and rigid structural sections cannot be chosen if their crosswise dimension unduly covers a print to interfere with guiding it into place for trimming. In addition to resisting direct forces incidental to its function, the form must otherwise be stable.

Blade 6, detailed in cross section in FIG. 21, is composed of three parts, permanently joined together by thermosetting plastic cement or by other means. It makes use of a light metal such as aluminum, yet has the cutting quality of tool steel; and its design takes into account the fact that light metals have a coefficient of thermal expansion of about twice that of steel. Since only the cutting edge need resist abrasion and wear, the tool steel part of the blade is confined to the outer part 56-1. The unit of greatest area is the central part 57. Largely by itself but also in combination with the other units which make up the blade, this central part 57 provides the stiffness in the blade. Having the greatest area, it is made of light alloy such as aluminum or magnesium, but chosen also with regard for stiffness. The inner part 58 is of steel; this need not be hard, but should have the same cross section and the same coeflicient of expansion as outer part 56. In central part 57 it is preferable that the same cross sectional area appear on each side of a vertical plane through its middle. In this construction bending forces resulting from differential thermal expansion on one side of the neutral axis are counterbalanced by like forces on the other side. The result is a blade of suitable rigidity that is thermally stable as to straightness, with a weight of about one-half that of an all-steel blade ofsimilar rigidity.

Retractable gage pin.-Trimmers of this type are used for cutting prints to many different dimensions. For this purpose a combined scale and squaring guide 59 is adjustably secured to board 1. A large part of the trimming work is confined to a few common dimensions such as 8 /2 inches and 11 inches. It has been common to mark prominently such dimensions on trim guide 59 so as to call the operators attention to them.

The present trimmer is provided at each such dimension with a positive stop in the form of a gage pin 60 shown in FIGS. 22, 23, and 24. An important feature of this gage pin 60 is its retractable nature. Quickly and easily, with but a slight motion of the operators hand above or below board 1, it is placed out of or brought into use. Gage pin 69 slides freely in the hole provided for it in gage pin body 61, but is kept at whatever position is given it, by retaining ball 62 acting against the wall of that hole due to retaining ball spring 63. The mouth of the hole in pin 60 which carries ball 62 and spring 63 is slightly deformed so as to retain ball 62 against loss when pin 60 is out of body 61. Body 61 is shouldered to assist in retaining it properly in board 1 with its upper surface flush with board 1. A saw out forms an integral spring 64 from the body 61. This is given a form to increase the friction between body 61 and its hole in board 1 so as to prevent accidental movement. The hole in body 61 which carries pin 60 is eccentric from body 61. This provides for minute adjustment of the actual distance from the gaging side of gage pin 57 to the cutting edge of the trimmer. To effect this adjustment, the top of body 61 is slotted so that body 61 may be turned conveniently with a screwdriver.

When using two gage pins 60 in trimming the two dimensions of a number of prints, the pin 60 at the longer dimension may remain protruding as this in no way interferes with use of the other pin 60 at the shorter dimension. Only the pin 60 at the shorter dimension need be manipulated, and this need not be moved in or out except to change it when going from one to the other of the dimensions.

Design for foldability.-Blocks 14 as shown in FIG. 3 are so located on base 4 as to permit blade 8 to clear them when it is folded against base 4. It is a simple matter to disengage stop 11 from board 1; blade 8 is pushed rearward when, under the action of spring 19, board 1 will come up above its normal terminal up position. This automatically causes sector 52 to disengage from board 1 and drop to the position it has in FIG. 3 Where it is clear of any interference with board 1 in any position. Board 1 may now be pressed down into a space of minimum height indicated by the diagram in FIG. 4. Only a few seconds are required for the entire disassembly.

Re-assernbly is similarly a speedy job. After raising board 1 to above the normal terminal position for it, blade 8 is turned rearward and held there While sector 52 islifted to a position where its free end will enter between rollers 48 and 49, whereupon board 1 is lowered and blade 8 is released so as to contact blade 6 with bumper 15 above blade 6. Then board 1 may be released to the action of spring 21 and the trimmer will be ready for work.

The construction makes it possible to ship the trimmer fully adjusted, in a package of minimum size, and without presenting the customer with any assembly problem worth mentioning. If the trimmer is shipped with board return spring 21 adjusted, the shipping case will hold board 1 against the action of that spring. For storage purposes it may be desired to have board 1 collapse under its own weight into the form indicated in the diagram of FIG. 4. To do this it is a simple matter to relax spring 21 by turning adjustment screw 26 after noting on its scale the proper position for tension against board 1. This'speeds the re-establishment of proper adjustment of screw 26 when next the trimmer is put into use.

When adjusting screw 26 it is convenient to open the trimmer into the position shown in FIG. 3. The trimmer will stand of itself in this positon as the forward edge of board 1 is formed to bear against strip 3 as shown in that figure.

While the invention has been described and illustrated in its several preferred embodiments, and has included certain details, it should be understood that the invention is not to be limited to the precise details herein illustrated and described since the same may be carried out in other ways falling within the scope of the invention as claimed.

I claim:

A blade for attachment to a cutting board comprising a lamination of two similar outer relatively thin strips of steel having similar coefficients of expansion under heat, one of said strips having a cutting edge thereon, and a relatively thicker intermediate strip of substantially lighter metal interposed between said outer strips and connected thereto to provide maximum rigidity in said blade for a given weight of material and to prevent said blade from warping from temperature changes.

References Cited by the Examiner UNITED STATES PATENTS 1,730,821 10/1929 Kempter 83698 X 2,588,809 3/1952 Demarest 83610 FOREIGN PATENTS 8,707 11/ 1915 Great Britain.

WILLIAM W. DYER, JR., Primary Examiner.

WILLIAM S. LAWSON, Examiner. 

